<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
      .a{
        width:200px;
        height:100px;
      }
      .b{
        padding:20%;
        background-color:red;
      }
    </style>
</head>
<body>
  <div class="a">
    <div class="b">
    </div>
  </div>
</body>
</html>